Facet junction engineering for enhanced SERS activity of Ag/Cu₂O composite substrates

Featuring ultra-high sensitivity and molecule-specific detection ability, surface-enhanced Raman scattering (SERS) is suitable for the rapid sensing of trace-level chemicals in biological, environmental, and agricultural samples. Although crystal facet junction engineering is a powerful tool to manipulate the optoelectronic properties of semiconducting materials, its correlation with the SERS sensing activity of noble metal/semiconductor composites has still not been clarified. In this work, Ag was deposited on Cu₂O nanocrystals enclosed by different facets, including {100} (cube), {111} (octahedron), and {100}/{111} (truncated octahedron), and a detailed study of their SERS performance was carried out. It was found that Ag/truncated-octahedral Cu₂O (Ag/Cu₂O(J3)) exhibited superior SERS activity to Ag/cubic Cu₂O (Ag/Cu₂O(C)) and Ag/octahedral Cu₂O (Ag/Cu₂O(O)). The {100}/{111} facet junction in Cu₂O can promote the separation and transfer of photogenerated charge carriers, which is beneficial for enhancing SERS sensing performance. Moreover, Ag/Cu₂O(J3) has a higher content of oxygen vacancies, providing extra interfacial charge-transfer pathways to the analyte, which also contribute to improving the SERS activity. The low detection limit of Ag/Cu₂O(J3) was 1 × 10^-11 M for 4-nitrobenzenethiol, two orders of magnitude lower than that of Ag/Cu₂O(C) and Ag/Cu₂O(O). In addition, Ag/Cu₂O(J3) could detect CV and R6G at concentrations down to 1 × 10^-10 M and 1 × 10^-8 M, respectively. The findings of this work can provide insightful information for designing metal/semiconductor substrates toward SERS sensing application by regulating the crystal facet junction.